Qt signals & slots mechanism

 #include <QObject>
 
 class Counter : public QObject
 {
     Q_OBJECT
 
 public:
     Counter() { m_value = 0; }
 
     int value() const { return m_value; }
 
 public slots:
     void setValue(int value);
 
 signals:
     void valueChanged(int newValue);
 
 private:
     int m_value;
 };

 void Counter::setValue(int value)
 {
     if (value != m_value) {
         m_value = value;
         emit valueChanged(value);
     }
 }

     Counter a, b;
     QObject::connect(&a, SIGNAL(valueChanged(int)),
                      &b, SLOT(setValue(int)));
 
     a.setValue(12);     // a.value() == 12, b.value() == 12
     b.setValue(48);     // a.value() == 12, b.value() == 48